Rock Bolting Method and Rock Bolt

ABSTRACT

A method for providing a rock bolt ( 2 ) for rock bolting, wherein a rock bolt ( 2 ) having a tubular bolt portion ( 4 ) with a closed cross section is inserted and expanded for bearing contact in a bore hole ( 3 ) in a rock structure ( 1 ). The length of the tubular bolt portion ( 4 ) is determined such that maximum frictional force at pulling inside a bore hole ( 3 ) of a rock bolt with a tubular bolt portion with this length is below a predetermined maximum value. The invention also concerns a rock bolt.

FILED OF THE INVENTION

This invention concerns a method in rock bolting according to the preamble of claim 1. The invention also concerns a rock bolt according to the preamble of claim 6.

DESCRIPTION OF BACK GROUND ART

From U.S. Pat. No. 4,459,067 (Atlas Copco AB) is previously known a rock bolt having a tubular bolt portion, which has a closed cross section and which is intended to be expanded for bearing against the inside of a bore hole in a rock structure.

In the expanded state of the tubular bolt portion, the previously known rock bolt is capable of supporting loads in order to i.a. stabilize a rock face belonging to a rock structure in connection with tunnelling, mining, drifting and the like.

AIM AND MOST IMPORTANT FEATURES OF THE INVENTION

It is an aim of the present invention to further develop a method in rock bolting, wherein rock bolts of the general kind which is described in the above mentioned U.S. Pat. No. 4,459,067 are used.

It is also an aim of the invention to provide a rock bolt as initially stated which allows secure and versatile use at the same time as it provides good economy.

These aims are achieved in a method according to the invention by determining the length of the tubular bolt portion such that the maximum fictional force between parts of the tubular bolt portion that bear against the inner wall of a bore hole, upon pulling inside this bore hole, is limited so that it is below a predetermined maximum value.

Hereby it is achieved that a rock bolt having a tubular bolt portion can be manufactured and dimensioned such that great pulling forces, which a rock bolt typically can be subjected to at powerful but in space limited movements in the rock structure it is intended to stabilize, does not cause breakage of the rock bolt but instead results in controlled sliding of the tubular bolt portion, a limited distance inside the bore hole, whereupon the rock bolt is again capable of resisting essentially the same load as prior to the sliding, so that such movements in the rock structure will be tolerable.

The advantages of the invention are besides that rock bolt failure is avoided also that the number of rock bolts per surface unit can be reduced, and that the dimensions of rock bolts that are used can be reduced, since the intended and resulting yielding thus allows rock movements in a controlled manner. Thereby the rock structure does not have to be stabilized in an exaggerated and a more costly manner.

It is preferred that the length of the tubular bolt portion is determined as the result of at least one measurement, preformed during pulling of at least one rock bolt having a tubular bolt portion of a predetermined in at least one bore hole in the same or in a corresponding rock structure.

Starting out from values resulting from this drawing measurement or these drawing measurements, the rock bolts to be used at the stabilization can thereupon be produced and adequately dimensioned with respect to the length of the tubular bolt portion, so that the intended sliding at intended loads is achieved.

It is preferred that said predetermined value is the yield point load for the tubular bolt portion, which means that the material is optimized to the existing conditions in an advantageous way. The yield point load for the tubular bolt portion of a rock bolt intended here can be determined by a person skilled in the art through in per se previously known load tests.

It is preferred that a number of drawing tests, as for example 5-10 drawing tests, are performed, in order to obtain more reliable values for dimensioning. Starting out from such a plurality of drawing test results, the distribution in the results is considered in order to determine the marginal to the yield point when the length of a tubular bolt portion which is to be used for each rock bolt, is to determine.

At greater distribution, a greater margin to the yield point is typically necessary. In that case it could be suitable to dimension a rock bolt such that, starting from the mean value of the drawing test, maximum calculated frictional force is as an example 60-70% of the yield point load. At smaller distribution, which also is to considered as normal distribution, starting from the mean value of the drawing tests, maximal calculated fictional force can be 80-90% of the yield point load. It shall be noted that this is exemplary and that it is within the scope of the invention that other values can be considered.

It is also preferred that the length of the tubular bolt portion is determined and dimensioned such that said frictional force exceeds a predetermined minimum value in order to avoid unnecessary sliding movements at less powerful rock movements.

The corresponding advantages are achieved through the invention through a rock bolt having corresponding features. Through an inventive rock bolt, great and sudden rock movements and be controlled. After a sliding movement, the capability of the rock bolt to take up load is essentially the same as it was before the sliding movement.

A device for performing said method includes means for engagement with a rock bolt having a tubular bolt portion inserted into and expanded inside a bore hole, means for applying a pulling force to said rock bolt, means for detecting sliding of said rock bolt as a result of application of said pulling force and means for detecting and measuring the pulling force that corresponds to the detected sliding.

This device allows effective determining of the effective length of a rock bolt which has the ability of yielding through sliding at greater movements in the rock structure.

The invention will now be described further with the aid of embodiments and with reference of the annexed drawings.

BRIEF DESCRIPTION OF DRAWING On the annexed drawing:

FIG. 1 shows a section through a rock structure with a rock bolt according to the invention,

FIG. 2 shows a device for performing a method according to the invention in connection with a rock bolt, and

FIG. 3 shows a sequence in order to illustrate the method in rock bolting according to the invention.

DESCRIPTION OF EMBODIMENT

In FIG. 1, reference number 1 relates to a rock structure, wherein a rock bolt 2 is inserted inside a bore hole, the inner wall of which is referenced 3. The rock bolt 2 includes a tubular expandable bolt portion 4 and a tension rod 5 which adjoins to the tubular bolt portion 4 over an adapter sleeve 6. On FIG. 1 is shown a tension rod in the form of a number of tubular sections 5′, in pairs joined with the aid of a joining sleeve 5″. This arrangement allows the use of rock bolts having a great total length also in narrow drifts, which otherwise would limit the length of a tension rod in one piece. Inside, most inwardly, in the bore hole 3 the tubular bolt portion 4 is completed with a terminal sleeve 7.

At the outside of the bore hole 3 a rock plate 8 is as usually applied, which through preloading of the tension rod 5 exerts an inwardly directed stabilizing pressing force against an outer part of the rock structure 1.

In FIG. 1, 9 indicates a crevice in the rock structure in order to somewhat illustrate the inherent instability of the rock structure 1. According to the invention, movements in the rock structure, which tend to displace the rock plate 8 axially outwards, that is to the right in FIG. 1, and that subject the rock bolt to a sufficiently high force, result in a sliding of the tubular bolt portion 4 against the inner wall of the bore hole 3. In particular this occurs if the force exerted on the rock bolt by the moveable portion of the rock structure, exceeds a minimum value.

This way a controlled yielding of the rock bolt occurs through the sliding movement, such that the rock bolt thereby is capable of resisting great forces and movements of the structure without failing.

In FIG. 2 is shown a device 14 for measuring frictional forces that are present in a certain type of expandable rock bolts in connection with a certain contracture. This device is intended for dimensioning of rock bolts according to the invention and in particular the length of the tubular bolt portion.

According to the invention, a process is undertaken in order to determine the effective length of a tubular bolt portion of a rock bolt before the positioning of a number of rock bolts in a rock structure in order to stabilize the rock face, as an example in tunnelling or drifting in mining.

The measurement is made such that a test specimen in the form of a rock bolt 10 having a tubular bolt portion of a predetermined length is inserted inside a bore hole 12 in the rock structuring 13 in question. The length of the tubular bolt portion is suitably chosen such that sliding of the rock bolt is safely achieved at a force that is below the yield point of the tubular bolt portion. If the yield point would be reached before sliding is obtained, a new rock bolt with a shorter tubular bolt portion 11 will be selected.

The device for measuring 14 includes a tension rod 15 for adjoining to the expandable tubular bolt portion, a supporting portion 16 for applying against the side of the rock structure, an engagement portion 17 for the engagement with the tension rod 15 and a pulling mechanism 18 which, for example with hydraulic means, applies a pulling force to the rock bolt 9. A control unit 19 senses the applied pulling force and detects when sliding occurs, and at which pulling force this occurs.

Starting from the results of the measurement, which can be transformed to fictional force per length unit tubular bolt portion, rock bolts can be dimensioned with adequate length of tubular bolt portion so that a pulling force corresponding to sliding for the chosen rock bolt with a tubular bolt portion of selected length as an example corresponds to about 60-about 90% of said yield point load. Selection of an adequate length can as an example be made practically through lists, diagrams or the like.

In FIG. 3 is illustrated a sequence for carrying out the method according to the invention.

Position 20 indicates the start of the sequence.

Position 21 concerns inserting and expanding a tubular rock bolt having a certain predetermined length in a predrilled bore hole in a rock structure.

Position 22 concerns axially pulling of the rock bolt in a direction outwardly.

Position 23 concerns detecting of sliding of the rock bolt and the pulling force, at which sliding occurs.

Through the interrupted line 26 is indicated that a plurality of drawing tests can be performed.

Position 24 concerns calculating of the length of the tubular bolt portion for a rock bolt, either through manual control in tables, diagrams etc. or automatically through a circuit which is connected to the control unit. This is performed starting from the drawing test or the drawing tests and associated detections performed in positions 22-24.

Position 25 concerns termination of the sequence.

The invention can be modified within the scope of the claims. Thus, the device for performing the method can be designed otherwise as long as it includes means for exerting a pulling force, detecting sliding and measuring pulling force at sliding.

It is preferred that the yield point is used as reference when dimensioning of rock bolts according to the invention. It is, however, not excluded that also other values that are significant for a rock bolt can be used for dimensioning. As an example the break load of the rock bolt could be used, wherein the tubular portion of the rock bolt could be dimensioned to have a length corresponding to a frictional force which is a certain determined part of the break load. It is also not excluded that dimensioning starts from the fact that the fictional force is to comprise a certain determined part of a significant value of the strength of the rock.

The rock bolt according to the invention could be of different embodiments. It is for example not excluded that a rock bolt according to the invention includes more than one tubular bolt portion arranged one after the other. 

1. Method for providing a rock bolt (2) for rock bolting, wherein a rock bolt (2) having a tubular bolt portion (4) with a closed cross section is inserted and expanded for bearing contact in a bore hole (3) in a rock structure (1), characterized in that the length of the tubular bolt portion (4) is determined such that maximum frictional force at pulling inside a bore hole (3) of a rock bolt with a tubular bolt portion with this length is below a predetermined maximum value.
 2. Method according to claim 1, charactrized in that the length of the tubular bolt portion (4) is determined starting out from a measurement or measurements preformed at pulling of at least one rock bolt (10) with a tubular bolt portion (11) of predetermined length in a bore hole (12) in the same or in a corresponding rock structure.
 3. Method according to claim 1, characterized in that said predetermined value is the yield point load of the tubular bolt portion.
 4. Method as claimed in claim 3, characterized in that the length is set such that said maximum calculated frictional force is set to about 60-about 90% of said yield point load.
 5. Method according to claim 1, characterized in that said length is determined such that minimal frictional force in a bore hole of the rock bolt (2) with a tubular bolt portion (4) with this length exceeds a predetermined minimum value.
 6. Rock bolt (2) including a tubular bolt portion (4) with a closed cross section for insertion and expansion for bearing contact in a bore hole (3), characterized in that the length of the tubular bolt portion (4) is dimensioned such that maximal frictional force when pulling inside a bore hole of a rock bolt with a tubular bolt portion with this length, is below a predetermined maximum value.
 7. Rock bolt according to claim 6, characterized in that the length of the tubular bolt portion (4) is determined starting out from at least one measurement preformed when pulling of at least one rock bolt having a tubular bolt portion of a determined length in a bore hole in the same or in a corresponding rock structure.
 8. Rock bolt according to claim 6, characterized in that said predetermined value is the yield point load of the tubular bolt portion.
 9. Rock bolt according to claim 8, characterized in that the length is dimensioned such that the said maximum frictional force comprises about 60-about 90% of said yield point load.
 10. Rock bolt according to claim 6, characterized in that said length is dimensioned such that minimal frictional force inside a bore hole for a rock bolt (2) with a tubular bolt portion (4) of this length exceeds a predetermined minimum value.
 11. Method according to claim 2, characterized in that said predetermined value is the yield point load of the tubular bolt portion.
 12. Method according to claim 11, characterized in that the length is set such that said maximum calculated frictional force is set to about 60-about 90% of said yield point load.
 13. Method according to claim 2, characterized in that said length is determined such that minimal frictional force in a bore hole of the rock bolt (2) with a tubular bolt portion (4) with this length exceeds a predetermined minimum value.
 14. Method according to claim 3, characterized in that said length is determined such that minimal frictional force in a bore hole of the rock bolt (2) with a tubular bolt portion (4) with this length exceeds a predetermined minimum value.
 15. Method according to claim 4, characterized in that said length is determined such that minimal frictional force in a bore hole of the rock bolt (2) with a tubular bolt portion (4) with this length exceeds a predetermined minimum value.
 16. Rock bolt according to claim 7, characterized in that said predetermined value is the yield point load of the tubular bolt portion.
 17. Rock bolt according to claim 16, characterized in that the length is dimensioned such that the said maximum frictional force comprises about 60-about 90% of said yield point load.
 18. Rock bolt according to claim 7, characterized in that said length is dimensioned such that minimal frictional force inside a bore hole for a rock bolt (2) with a tubular bolt portion (4) of this length exceeds a predetermined minimum value.
 19. Rock bolt according to claim 8, characterized in that said length is dimensioned such that minimal frictional force inside a bore hole for a rock bolt (2) with a tubular bolt portion (4) of this length exceeds a predetermined minimum value.
 20. Rock bolt according to claim 9, characterized in that said length is dimensioned such that minimal frictional force inside a bore hole for a rock bolt (2) with a tubular bolt portion (4) of this length exceeds a predetermined minimum value. 